1. Field of the Invention
This present invention relates to an image forming apparatus and a control method thereof.
2. Description of the Related Art
Currently, some of image forming apparatuses such as printers, copiers, scanners, and multifunction peripherals having their functions have a function to form images on both sides of a paper sheet. It is essential for those image forming apparatuses to adjust misregistration of the images formed on the front and the back of the paper sheet, respectively, in order that positions to form the images on the front and the back of the paper sheet are aligned.
For example, Japanese Patent Application Laid-Open Publication No. 2000-108358 discloses an ink-jet printer which prints on the front face and the rear face of continuous roll paper. The ink-jet printer prints a visual confirmation pattern composed of dots, which are uniformly scattered within a belt-shaped area extending in a width direction of the continuous roll paper, at a print start time, at a print end time, and during an optional print period so as to monitor a print state on the continuous roll paper by a print state of the visual confirmation pattern.
Japanese Patent Application Laid-Open Publication No. 5-8380 discloses a registering device which takes a photograph of top-side and rear-side registration marks formed on the top surface and the rear surface of printing paper, respectively, from the top surface of the printing paper for obtaining image information on the top-side and rear-side registration marks, calculates misregistration between the top-side and rear-side registration marks based on the image information, and gives a control signal to an image forming device so as to reduce the misregistration.
Japanese Patent Application Laid-Open Publication No. 5-254105 discloses a registration mark inspecting device including: two objectives which form front and rear registration mark images of a both-sided print and are arranged facing each other; one ocular for observing the front and rear registration mark images; and transmission optical systems for transmitting the respective images of the front and rear registration mark images formed by the respective objectives to the ocular by interconnecting respective optical axes of the two objectives to an optical axis of the ocular in a coaxial relation. One of the transmission optical systems has a reverse optical part which reverses a surface normal to the optical axis of one of the front and rear registration mark images, the registration mark image which is formed by the objective corresponding to the transmission optical system, in one direction by a reflection.
Japanese Patent Application Laid-Open Publication No. 2008-275991 discloses an image forming apparatus which reads a recording sheet of which front and back image position adjusting marks are formed on both sides, respectively, by irradiating the recording sheet with light from one side of the both sides of the recording sheet, detects the front image position adjusting mark on the one side thereof and the back image position adjusting mark on the other side thereof which is read through the one side thereof, and determines an adjustment value, so that the detected image position adjusting marks on the both sides thereof agree with each other.
However, in the related arts mentioned above, in order to check the misregistration caused on the front and the back of a paper sheet, a dedicated paper sheet for misregistration check, namely, the dedicated paper sheet of which marks (registration marks) for checking the misregistration of images formed on the front and the back of a paper sheet are formed on the front and the back, is needed to output. On the dedicated paper sheet for misregistration check, only the registration marks are formed in many cases. Therefore it is difficult for a user thereof to find exactly what kind of misregistration occurs to the images formed on the front and the back of a paper sheet.
Furthermore, the misregistration is often checked when a user notices the misregistration. Consequently, it is difficult to notice an occurrence of the misregistration in its early stage, and paper sheets are wasted when the paper sheets having the misregistration are output.